a. Field of the Invention
This invention relates to electrical power generators, and more particularly, to voltage regulation controls for wound rotor synchronous generators for use in aerospace applications, such as aircraft.
b. Description of Related Art
FIG. 1 schematically illustrates a prior art integrated aircraft power generating system 10 for generating three-phase 400 Hz 115 volt alternating current. The integrated power generating system 10 is driven by a power takeoff 12 from an aircraft propulsion engine which varies in speed during operation of the aircraft. The power takeoff 12 is coupled to a constant speed drive transmission 14 which functions to produce a constant speed output on shaft 16 while the rotational speed of the power takeoff varies as is commonly known to those skilled in the art. The integrated power system 10 comprises a permanent magnet generator 18, wound field exciter 20 and a main generator 22 known to those skilled in the art. Each has a rotor mounted on shaft 16 and are all mounted in a housing not illustrated.
Permanent magnet generator 18 has a permanent magnet rotor 24 mounted on the shaft 16. The stator 26 of the generator 18 outputs alternating current which is rectified by rectifier 28 to produce field excitation current which is applied to the stator 30 of the wound field exciter 20. The rotor 32 of exciter 20 outputs alternating current which is rectified by rectifier 34. Rectified current from rectifier 34 is applied to the field windings of the rotor 36 of the main generator 22 to control the output voltage of the main generator. The stator 38 outputs three-phase 400 Hz 115 volt alternating current for use in powering the various electrical loads of the aircraft.
Normally, the voltage regulator for the wound rotor synchronous generator 22 controls generator output voltage by controlling the field current of exciter 20 using the generator output voltage potential as a feedback signal to the regulator. At low engine speeds, which translates into low generator speeds, however, the generator 22 cannot supply the load requirements because the field of exciter 20 reaches current saturation and prevents voltage regulation of the generator 22. Low speed conditions exist in an aircraft, for example, during start-up of the engines, aircraft taxi on the runway and landing. As such, during low engine speeds, the aircraft must rely on alternate power sources, such as an auxiliary power unit, to meet the power requirements of the aircraft.